The entire disclosure of Japanese Patent Application No. 2001-351159 filed on Nov. 16, 2001 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to a spindle apparatus of a machine tool, such as a machining center.
2. Description of Related Art
A built-in motor type spindle apparatus, in which a rotor of a motor is mounted on a main spindle, a stator of the motor is mounted on an inner peripheral portion of a spindle case, and the motor is built into the spindle case, is well known as a spindle apparatus of a machine tool, such as a machining center.
For such a built-in motor type spindle apparatus, various cooling mechanisms have so far been proposed in order to avoid the heat deformation of the main spindle due to a temperature rise caused by heat generation of the built-in motor (spindle motor), in addition to conduction and radiation of the rolling frictional heat of bearings.
For example, Japanese Unexamined Patent Publication No. 2000-15541 discloses a spindle apparatus in which a continuous helical groove is provided on an axial outer peripheral surface of a main spindle from a front portion to a rear portion thereof, and a cooling liquid is circulated through the helical groove and brought into contact with inner rings of a front bearing and a rear bearing and the interior of a rotor of a spindle drive motor to cool the inner rings and the rotor directly and also cool the main spindle itself.
Japanese Unexamined Patent Publication No. 2000-52188 discloses a built-in motor type spindle apparatus in which a ring-shaped heat radiation fin member rotating integrally with a main spindle of the spindle apparatus is mounted on the main spindle, and a cooling air passage for blowing cooling air against the heat radiation fin member is formed in a spindle case.
However, the former spindle apparatus requires a measure for preventing leakage of the cooling liquid, making the apparatus complicated and inducing a cost increase. With the latter spindle apparatus, cooling air is relatively higher in density and higher in fluid resistance than other gases, thus causing a considerable power loss to the main spindle as a rotating part and requiring a great motor power.
The present invention has been proposed in light of the above-mentioned problems with the conventional spindle apparatuses. Its object is to provide a spindle apparatus having a simple structure which minimizes a power loss of a main spindle, can decrease a motor power, and produces a cooling effect.
According to the present invention, designed to attain the above object, there is provided a spindle apparatus of a machine tool, comprising a main spindle rotatably supported in a housing via bearings, and wherein means for circulating a gas lower in density and fluid pressure than air is provided in the housing, and the main spindle is rotated in an atmosphere of the circulating gas. Thus, the power loss of the main spindle can be minimized, the motor power can be decreased, and a cooling effect is obtained. Furthermore, some leakage of the circulating gas can be allowed, so that the sealing structure can be simplified.
In the spindle apparatus, a sealing gas may be supplied to a gap between the housing and the main spindle to isolate an interior of the housing from an outside. In this case, the circulating gas, which has been supplied to the gap on the outer periphery of the main spindle (within the housing), can be prevented from leaking to the outside through the gap between the housing and the main spindle. Besides, a coolant or swarf can be prevented from entering the gap on the outer periphery of the main spindle from the outside through the gap between the housing and the main spindle.
In the spindle apparatus, a sealing gas separator for separating and discharging the sealing gas may be interposed in a circulation system for the circulating gas. In this case, a sufficient amount of the circulating gas is always circulated in the gap on the outer periphery of the main spindle (within the housing).
In the spindle apparatus, the circulating gas may be helium. In this case, advantages over the conventional apparatus in terms of cost and safety are obtained, in addition to the aforementioned effects.
In the spindle apparatus, a rotor of a motor may be mounted on the main spindle, a stator of the motor may be mounted on an inner peripheral portion of the housing, and the motor for the main spindle may be built into the housing. In this case, a temperature rise in the main spindle due to heat generation of the so-called built-in motor can be effectively suppressed.